The reductive dechlorination potential of an in vitro system (rat liver homogenate) was investigated to aid in elucidating the role of metabolic dechlorination in toxicity. The chlorinated polycyclodiene pesticide dieldrin was chosen as a model substrate due to the availability of the various dechloro products which might be expected. Preliminary findings suggest that dieldrin may require NADPH and anaerobic conditions to effect dechlorination.